The invention relates to a cylinder head gasket with a gasket plate comprising several sheet metal layers and having at least one combustion chamber through-opening, a first sheet metal layer being provided with a ring-shaped sealing element surrounding the combustion chamber through-opening, projecting in the direction towards an adjacent, second sheet metal layer and resting against the second sheet metal layer at least when the cylinder head gasket is installed, the sealing element being capable of undergoing elastic deformation perpendicularly to the gasket plate, and a metallic spacer ring covered by the two sheet metal layers being secured to one of the two sheet metal layers, namely by a ring-shaped weld seam, radially adjacent to the sealing element for delimitation of the deformation of the sealing element, the spacer ring facing the other sheet metal layer and surrounding the combustion chamber through-opening. In particular, the sealing element is a bead which can have the configuration of a so-called full bead of approximately circular-arc-shaped cross section or a so-called half beadxe2x80x94the cross section of a half bead has approximately the shape of a step or a Z pulled almost flat. In principle, however, other bead-shaped sealing elements are also conceivable, which undergo elastic deformation perpendicularly to the plane of the gasket plate when clamping the cylinder head gasket and during operation of the engine. Mention is also made of the fact that the above-mentioned spacers are often also referred to as stoppers.
The invention further relates to a process for the manufacture of such a cylinder head gasket, more specifically, a process for producing the weld seam by means of which the spacer ring is joined to the sheet metal layer carrying it.
EP-O 486 817-B1 discloses a cylinder head gasket with a gasket plate formed by a sheet metal layer and with a sheet metal sealing ring secured to this sheet metal layer by laser spot welding, the sheet metal sealing ring directly surrounding a combustion chamber through-opening of the sheet metal layer and serving to delimit the elastic deformation of a bead (particularly when tightening the cylinder head screws), which is formed in the sheet metal layer forming the gasket plate, surrounds the combustion chamber through-opening and the sealing ring in the shape of a ring and has a height which is somewhat larger than the thickness of the sealing ring. The weld spots serving to secure the sealing ring to the sheet metal layer forming the gasket plate are configured and arranged such that their diameter is somewhat larger than their distance from center to center, measured in the circumferential direction of the sealing ring, so that weld spots adjacent to one another overlap one another. The laser beam is directed onto the upper side, i.e., the visible side of the sealing ring so that the ring surface of the sealing ring resting against the cylinder head (or possibly against the engine block) has a scale-like surface structure.
Such a weld seam consisting of a series of weld spots has quite a number of disadvantages: Firstly, it is not ensured that the joint between the sheet metal sealing ring and the sheet metal layer carrying it is really gas-tight, and, secondly, the above-mentioned scale-like surface structure of the weld seam can have disadvantageous effects: Owing to the engine dynamics, a bead is periodically elastically flattened during operation of the engine and then rises again, and this working of the bead automatically results in an oscillating, radial pushing movement with respect to the combustion chamber axis between the sheet metal ring in question and that surface against which the sheet metal ring is supported during operation of the engine. The rough upper side of the sheet metal ring then results in the risk that the sheet metal ring will be destroyed or released from the sheet metal layer carrying it, and if parts of the sheet metal ring drop into the combustion chamber this can result in the engine being destroyed.
Cylinder head gaskets of the kind mentioned at the outset are known from WO 97/31204 of the company Elring Klinger GmbH, wherein the spacer ring is secured in a gas-tight manner using a continuous wave laser to the sheet metal layer carrying the spacer ring by a continuous, ring-shaped laser weld seam which is closed within itself and is uniform except for an overlapping area. Areas at the start and end of the weld seam liexe2x80x94in the plan view of the spacer ringxe2x80x94alongside one another or almost coincide in the above-mentioned overlapping area. The above-described disadvantages of the laser spot welding resulting from EP-0 486 817-B1 can be avoided with such a laser weld seam. On the other hand, it requires quite high expenditure to produce the continuous weld seam using a continuous wave laser, namely not only in view of the devices required therefor, but also because of the relatively high cycle time due to the duration of the welding operation in the series production of the cylinder head gaskets in question.
The object underlying the invention was to create a cylinder head gasket of the kind mentioned at the outset and to indicate a process for the manufacture thereof, thereby enabling at least the time expenditure for the production of a gas-tight weld seam joining the spacer ring to the sheet metal layer carrying it to be minimized.
This object is accomplished in accordance with the invention in that the spacer ring is secured in a gas-tight manner by electric pulse welding to the sheet metal layer carrying it by a continuous, ring-shaped, weld seam which is at least almost uniform overall and is closed within itself by the spacer ring and the sheet metal layer carrying it being clamped between a first electrode resting against the spacer ring and a counter electrode, and a current pulse adequate for the welding then being passed through a ring-shaped, sharp edge-like or bead-like contact projection of the first electrode pressed against the spacer ring, through the spacer ring, the sheet metal layer carrying it and through the counter electrode. The spacer ring and the sheet metal ring carrying it can only be clamped between the contact projection of the first electrode and the counter electrode.
A cylinder head gasket according to the invention produced in this way is then characterized by the spacer ring being secured in a gas-tight manner to the sheet metal layer carrying it by a continuous, ring-shaped, overall at least almost uniform weld seam made by electrowelding and closed within itself, which has on the free side of the spacer ring a ring-shaped, groove-like depression whose shape corresponds approximately to that of the contact projection of the first electrode.
The electric pulse welding by means of a ring-shaped, sharp edge-like or bead-like contact projection of an electrode makes very short cycle times possible (up to 60 or more weld seams can be produced per minute), as the welding energy normally stored capacitively in the welding device can be converted into a sufficiently high welding current concentrated through the contact projection and lasting only a few milliseconds, with a single current pulse being sufficient for the welding. Current pulses with a duration of only 12 to 15 milliseconds are given by way of example. In addition, the electric pulse welding devices available on the market enable the welding energy to be infinitely set and also constantly monitored by means of an electronic control.
Although it is preferable to press the one electrode with the sharp edge-like or bead-like contact projection against the spacer ring and to press the counter electrode with a flat end face against the sheet metal layer carrying the spacer ring, it is, in principle, also conceivable to press the one electrode with the sharp edge-like or bead-like contact projection against the sheet metal layer carrying the spacer ring or to provide both electrodes with ring-shaped, sharp edge-like or bead-like contact projections which are located opposite each other during the welding so that as short as possible a current path of the welding current through the spacer ring and the sheet metal layer results and the contact projections bring about an even stronger concentration of the welding current.
In principle, it is possible to weld a spacer ring as such to the sheet metal layer carrying it. However, above all, on account of the fact that the width of such spacer rings is relatively small, which also makes handling of these difficult, a different procedure is recommended: As such spacer rings have to be stamped out of a sheet metal layer anyhow, one can also weld a sheet metal disc whose outer diameter corresponds to the outer diameter of the later spacer ring to the sheet metal layer provided for carrying the spacer ring and then stamp an opening out of the disc in order to produce the spacer ring.